We just visited both waterparks with our 10 and 12yos in June. I would say it depends on how BRAVE your 9yo is. If they are a real thrill seeker, then Blizzard Beach. If they are a little more timid, then Typhoon Lagoon. You can't lose with either one though, they are both fun.
The lockers are all day, one price, you get a key on a wristband you keep with you. There are large and small lockers. We used one for the four of us and it was $8/day (plus a $5 deposit you get back when you return the key). For the life of me I can't remember if we had a large or small locker??